1. Field of Invention
This invention relates to methods and apparatus for adaptively controlling the state of charge of a battery array of a series hybrid electric vehicle.
2. Description of Related Art
The desire for cleaner air has caused various federal, state, and local governments to change their regulations to require lower vehicle emissions. Increasing urban traffic congestion has prompted a need for increases in public mass transit services. Many large cities use buses to transport people into, out of, and within traffic congested urban areas. Conventional buses use diesel powered internal combustion engines. Diesel engines produce emissions, including carbon monoxide, that contribute to air pollution. It is possible to refine cleaner diesel fuel. However, cleaner diesel fuel is more costly to refine and causes a corresponding increase in the cost of bus service.
Alternative fuels have been used to reduce emissions and conserve oil resources. Compressed natural gas has been used as an alternative fuel. Compressed natural gas does not produce as much power in conventional internal combustion engines as gasoline and diesel and has not been widely developed or accepted as an alternative to gasoline and diesel.
Additives have also been developed for mixing with gasoline to reduce emissions. Ethanol and MTBE have been added to gasoline to oxygenate the combustion of gasoline and reduce emissions of carbon monoxide. These additives, however, are believed to cause decreased gas mileage and, in the case of MTBE, to be a potential public health threat.
Electric vehicles have been developed that produce zero emissions. Electric vehicles are propelled by an electric motor that is powered by a battery array on board the vehicle. The range of electric vehicles is limited as the size of the battery array which can be installed on the vehicle is limited. Recharging of the batteries can only be done by connecting the battery array to a power source. Electric vehicles are not truly zero emitters when the electricity to charge the battery array is produced by a power plant that burns, for example, coal.
Hybrid electric vehicles have also been developed to reduce emissions. Hybrid electric vehicles include an internal combustion engine and at least one electric motor powered by a battery array. In a parallel type hybrid electric vehicle, both the internal combustion engine and the electric motor are coupled to the drive train via mechanical means. The electric motor may be used to propel the vehicle at low speeds and to assist the internal combustion engine at higher speeds. The electric motor may also be driven, in part, by the internal combustion engine and be operated as a generator to recharge the battery array.
In a series type hybrid electric vehicle, the internal combustion engine is used only to run a generator that charges the battery array. There is no mechanical connection of the internal combustion engine to the vehicle drive train. The electric traction drive motor is powered by the battery array and is mechanically connected to the vehicle drive train.
In present hybrid electric vehicles, the internal combustion engine and the generator are designed based to meet the peak power demands that the vehicle will encounter. The engine, the generator, and the electric motor must be "over-sized" to meet the peak power demand. The need to meet the peak power demand, as opposed to the average or normal power demand, causes an increase in the size and cost of the engine and the generator and a decrease in reliability and efficiency of the vehicle. Present series type hybrid electric buses, for example, include a 250 Hp diesel engine and a 250 Hp electric traction drive motor. The performance enhancement factor (PEF), the ratio of the power of the electric motor to the power of the internal combustion engine, for such a vehicle is 1. The present series type hybrid electric buses require a larger internal combustion engine in order to maintain the power output after the battery array has been substantially discharged. The internal combustion engine must also be large enough to run the generator to produce additional power for additional systems, such as heating the bus and running pumps and fans of cooling systems, that operate simultaneously.